1. Field of the invention
This invention relates to a semiconductor laser device with excellent low noise characteristics.
2. Description of the prior art
In general, semiconductor laser devices used in video disc players and other apparatuses are required to have excellent low noise characteristics. To meet this need for low noise characteristics, semiconductor laser devices employing the self-sustained pulsation phenomenon are often used. If the self-sustained pulsation phenomenon is employed, the oscillation spectrum has multiple longitudinal modes. Moreover, the spectral band width of each longitudinal mode becomes larger, thereby attaining low noise characteristics.
FIG. 11 shows a conventional semiconductor laser device. In this semiconductor laser device, the active layer has a quantum-well structure to decrease the oscillation threshold current. The semiconductor laser device can be produced as follows. On the plane of an n-GaAs substrate 91, an n-GaAs buffer layer 92, an n-AlGaAs cladding layer 93, an AlGaAs graded-index light guiding layer 94, an AlGaAs quantum-well active layer 95, an AlGaAs graded-index light guiding layer 96, a p-AlGaAs cladding layer 97, and a p-GaAs cap layer 98 are successively grown by molecular beam epitaxy. The Al mole fractions in the graded-index light guiding layers 94 and 96 are allowed to decrease gradually toward the active layer.
Next, the p-AlGaAs cladding layer 97 and the p-GaAs cap layer 98 are subjected to a photolithographic treatment to form a striped ridge portion 82 (width W.sub.1 =3 .mu.m) by a reactive ion beam etching technique. Then, an SiN.sub.x insulating layer 99 formed on the entire surface of the p-AlGaAs cladding layer 97 and the side faces of the p-GaAs cap layer 98, after which a p-type electrode 80 and an n-type electrode 81 are formed on the upper face of the p-GaAs cap layer 98 and the back face of the n-GaAs substrate 91, respectively, resulting in a semiconductor laser device.
In this semiconductor laser device, because the sum d of the thicknesses of the p-AlGaAs cladding layer 97 and the AlGaAs graded-index light guiding layer 96 on both sides of the striped ridge portion 82 is set at a large value, the difference in the effective refractive index between the region underneath the striped ridge portion 82 and the adjacent regions thereto becomes small. Furthermore, the oscillation threshold current can be reduced, because the active layer has a quantum-well structure. However, because the index guiding mechanism is weak in this semiconductor laser device, the characteristics of the laser beam obtained are similar to those of the laser beam emitted by a gain guiding mechanism. Therefore, the degree of astigmatism thereof is extremely high, being greater than 30 .mu.m.